Functional Component Driven Phase Stabilization and Defect Passivation toward Efficient and Air-Stable CsSnI₃...

All-inorganic CsSnI₃ has emerged as a promising candidate for a near-infrared emitter. However, its tolerance factor (t) resides near the lower limit of the structural stability range for perovskite crystals, resulting in a spontaneous phase transition under ambient air. Here, we developed a dual-ion substitution strategy via rational component engineering to address these issues. Systematic substitution of Cs⁺ with guanidinium (GA⁺), coupled with partial I^- replacement by thiocyanate (SCN^-), effectively optimizes t, stabilizing the B-γ phase for over 200 min in ambient air. Mechanistic studies reveal that SCN^- coordinates with Sn²⁺ to suppress oxidation, while -NH₂ in GA⁺ forms H-bonds with I that inhibit V_I formation. Consequently, near-infrared light-emitting diodes based on GASCN-incorporated CsSnI₃ exhibit a peak external quantum efficiency of 6.01% and an excellent operating lifetime of 2500 min. Importantly, we demonstrate the air-stable operation of such devices in practical applications including night vision, medical imaging, and nondestructive testing.